1. Field of the Invention
The present invention relates to variable device circuits and method for manufacturing the same, which can be formed by applying micromachining techniques or the like and which is suitable for radio-frequency variable components to be used in radio-frequency bands such as microwaves, semi-millimeter waves and millimeter waves.
2. Description of the Related Art
Recently, radio-frequency devices using micromachining techniques, so called RFMEMS (Radio Frequency Micro-Electro-Mechanical-Systems) devices, have been drawing attention. With this technique, high-aspect three-dimensional structures, hollow structures, variable structures and the like can be easily manufactured, thereby expectably allowing radio-frequency devices and circuits to be fabricated on low-priced silicon substrates with low cost, low loss, high isolation, and high-performance.
Also recently, RFMEMS radio-frequency variable circuits made up of RFMEMS active devices and RFMEMS passive devices, based on mechanical switches, have been in the spotlight as a key technology to next-generation radio-frequency modules that are required to operate on plural frequency bands. Further, RFMEMS radio-frequency variable circuits, when integrated in combination with existing CMOS high-frequency circuits, are expected to realize radio-frequency modules with higher functions, lower cost and lower size.
In addition, research and development of radio-frequency variable devices and circuits using micromachining techniques have been being advanced. For instance, Non-Patent Document 1 shown below, discloses a radio-frequency variable capacitor (hereinafter, referred to as first prior art) formed by using micromachining techniques.
The radio-frequency variable capacitor according to the first prior art is configured of a coplanar transmission line formed on a substrate, a bridge structure formed above a signal line of the coplanar transmission line, and a movable beam having an electrode connected from above the bridge through a pole.
Application of a driving voltage between the movable beam and the substrate causes the movable beam to be pulled toward the substrate by electrostatic force, so that the bridge which is connected thereto through the pole and provided on the coplanar transmission line is deformed. This deformation leads to a change in distance between the signal line of the coplanar transmission line and the bridge crossing thereover, which in turn results in a change in capacitance between the signal line of the coplanar transmission line and the bridge, hence, thereby acting as a variable capacitor. The change of the capacitance can be adjusted by the driving voltage applied to the movable beam.
Patent Document 1 shown below also discloses a radio-frequency variable device formed by using micromachining techniques (hereinafter, referred to as second prior art).
The radio-frequency variable device according to the second prior art is configured of a variable capacitor having a drive mechanism formed on the top face of the substrate using surface micromachining techniques, a fixed capacitor implemented by a pair of plural electrodes formed on the rear face of the substrate using bulk micromachining techniques, a switch formed on the top face of the substrate using surface micromachining techniques, and wiring lines for electrically connecting the switch, the variable circuit and the fixed capacitor to one another.
The fixed capacitor composed of a pair of plural electrodes is provided in plurality, and a fixed capacitor having a desired capacitance is selected by using the switch.
In the variable capacitor, a movable membrane and an upper electrode are provided above the wiring lines coupling to a lower electrode, in which application of a driving voltage between the lower electrode and the upper electrode causes the membrane to be deformed by electrostatic force, which in turn results in a change in distance between the upper electrode and the lower electrode so that the capacitance can change, thereby acting as a variable capacitor.
A fixed capacitor selected by the switch is electrically communicated with the variable capacitor, and the operating frequency of the device can be tuned stepwise by the selected fixed capacitor and moreover fine tuned by the variable capacitor.
The related prior arts are listed as follows: Japanese Patent Unexamined Publication (koukai) JP-2004-327877A, and a literature: D. Peroulis and P. B. Katehi, “Electrostatically-Tunable Analog RF MEMS Varactor with Measured Capacitance Range of 300%”, 2003 IEEE MTT-S International Microwave Symposium Digest, pp. 1793-1796, 2003.
The radio-frequency variable capacitor according to the first prior art is configured of the bridge and the movable beam having the electrode above a coplanar transmission line, in which the bridge is connected to the movable beam via through the pole. In this case, a complicated drive mechanism is required, and the variable capacitor is not easy to manufacture.
Also, since the radio-frequency variable capacitor has insecure factors in terms of mechanical strength, there is a need for preparing a large-sized movable beam to give enough electrostatic force to the movable beam. This would lead to increases in size of the device or the like, entailing a problem that integration of switches, fixed capacitors, and variable inductors becomes difficult in manufacturing.
Further, although the relative variability ratio of capacitance is large, the area of the bridge as well as the distance between the bridge and the signal line of the coplanar transmission line are limited in terms of the device structure and its manufacture, so that the resultant capacitance is also limited. As a result, it is difficult to form the bridge and the movable beam above a thick-film coplanar transmission line because of a large step gap. This leads to an issue that conductor loss of the coplanar transmission line is hard to reduce.
On the other hand, in the case of the radio-frequency variable device according to the second prior art, the switch and the variable capacitor are fabricated on the top face of the substrate by using surface micromachining, and the fixed capacitor is fabricated on the rear face of the substrate by bulk micromachining. This would leads to a complexity in the fabrication process.
Also, because of increases in insertion loss due to through-wiring that connect the top face with the rear face of the substrate, a deep digging process of the substrate is indispensable in the fabrication process of the fixed capacitor. This makes a weak point of the radio-frequency variable capacitor in terms of its manufacturing cost.
Furthermore, in a comb-tooth structure making up the fixed capacitor, there is a high possibility of short circuits between electrodes due to metallization. It is also difficult to provide a lid structure, which is necessary to obtain stable electrical characteristics, on the rear face on which the fixed capacitor is formed. Still more, because of the structure that has difficulty in forming a dielectric film, there is a limitation of capacitance.
Moreover, for the variable capacitor, there is a high possibility of short circuits between the upper electrode (movable electrode) and the lower electrode due to a high step gap, giving rise to a problem in operational reliability of the fixed capacitor and the variable capacitor.